A pump jack (also called oil horse, donkey pumper, rocking horse, beam pump, and jack pump, among other names) is the over-ground drive for a reciprocating piston pump in an oil well. Pump jacks are designed to mechanically lift liquid out of the well in cases in which insufficient bottom hole pressure exists for the liquid (typically an emulsion of oil and water) to flow to the surface.
A pump jack converts the rotary motion of an electric motor or internal combustion engine to a vertical reciprocating motion to drive the pump shaft. The motor of the pump jack runs a set of pulleys to the transmission, which drives a pair of cranks equipped with counterweights to assist the motor in lifting the heavy string of rods. The cranks raise and lower one end of an I-beam which is free to move on an A-frame or Samson post. On the other end of the beam is a curved metal box called a horse head or donkey head. A cable made of steel (or occasionally, fiberglass) called a bridle, connects the horse head to the polished rod, which is a piston that passes through a stuffing box. The polished rod has a close fit to the stuffing box, letting it move in and out of tubing without fluid escaping. The tubing is a pipe that runs to the bottom of the well through which the liquid is produced. The bridle follows the curve of the horse head as it lowers and raises to create a nearly vertical stroke. The polished rod is connected to a long string of rods called sucker rods, which run through the tubing to the down-hole pump, usually positioned near the bottom of the well.
A well may be lined with cement walls. Within the cement walls is the tubing, the down-hole pumps and the sucker rods. The down-hole pump is located at the bottom of the tubing. On the downward stroke, the plunger of the down-hole pump fills with oil, and on the upward stroke this oil is transported to the top of the well where it is extracted and put into barrels. The down-hole pump has two ball check valves: a stationary valve at bottom called the standing valve, and a valve on the piston connected to the bottom of the sucker rods that travels up and down as the rods reciprocate, known as the traveling valve. Oil-containing fluid enters from the formation into the bottom of the borehole through perforations that have been made through the casing and cement. When the rods at the pump end are traveling up, the traveling valve is closed and the standing valve is open (due to the drop in pressure in the pump barrel). Consequently, the pump barrel fills with the fluid from the formation as the traveling piston lifts the previous contents of the barrel upwards. When the rods begin pushing down, the traveling valve opens and the standing valve closes (due to an increase in pressure in the pump barrel). The traveling valve drops through the fluid in the barrel (which had been sucked in during the upstroke). The piston then reaches the end of its stroke and begins its path upwards again, repeating the process.
Pump jacks are typically used for low-volume marginal or “stripper” wells that produce no more than 15 barrels a day. The average stripper well produces only about 2.2 barrels per day. Depending on the size of the pump jack, anywhere from 5 to 40 liters of liquid may be delivered to the surface with each stroke. Although designed and used for low yield wells, pump jacks service 84 percent of U.S. oil wells and produce, in the aggregate, fully 18 percent of all US domestic oil. Keeping these pump jacks operating at peak performance is, therefore, vitally important. Significantly, it has become clear that early detection of anomalies in the operation of the pump jacks is key to identifying less than optimal operation or indeed predicting impending failure of the jack pump. In turn, such anomaly detection may allow for predictive maintenance to be carried out before failure or significant performance degradation.